1. Field of the Invention
The present invention relates to an ink composition suitable for recording with aqueous ink, especially for ink-jet printing. More particularly, the present invention relates to a water-resistant ink composition superior in water resistance, storage stability, ink delivery stability, and color development. The ink composition does not run even if exposed to wet once it has been printed and dried.
The present invention also relates to a writing instrument (such as ball-point pen, sign pen, marker pen, felt pen, and fountain pen) that employs aqueous ink. More particularly, the present invention relates to a writing instrument with a water-resistant ink composition which does not run even if exposed to wet once it has been written and dried.
2. Description of the Prior Art
The recent growing social interest in environmental protection has stimulated the switching over from solvent-based paints, coating materials, and dyes to water-based ones. This holds true of writing instruments which are used mostly in offices and homes where safety precedes.
For this reason, "aqueous" ball-point pens and "aqueous" ink for fountain pens have recently appeared on the market. These writing instruments need water resistance because they are used in places where water, coffee, juice, etc. are habitually had. There are few ware-resistant aqueous inks on the market. Although some claim to be water resistant, they easily run if exposed to wet immediately after writing.
A silicone compound has often been used to impart water resistance. It imparts good water resistance when applied from a solvent solution, but it is usually unstable in water. It may be slightly stable under weak acidic conditions but becomes unstable under alkaline conditions. It cannot be used in systems containing an aqueous dye which is usually alkaline.
There is proposed an aqueous composition for surface treatment in Japanese Patent Laid-open No. 279678/1994. It is a hydrolyzate of a reaction product from an aminotrialkoxysilane and an organic compound reactive with its amino group. It may be further reacted with an organometallic compound such as tetraalkoxysilane. Unfortunately, it is poor in water solubility because its amino group, which greatly contributes to water solubility, is blocked by the organic group. Common practice to eliminate this disadvantage is to add an anionic surface active agent as a stabilizer. Such an additive, however, sometimes produces an adverse effect on water resistance when added to aqueous ink. In addition, the solution containing it is not so good in long-term storage stability and unstable under alkaline conditions. Therefore, it is not an effective additive for aqueous ink compositions.
Ink-jet recording uses several methods for delivering ink (recording liquid). One method gives mechanical vibration or displacement to ink by means of electrostatic attraction or piezoelectric motion, thereby producing ink droplets. Another method heats ink to produce ink droplets by utilizing pressure resulting from foaming. Ink droplets thus formed emerge partly or entirely from the nozzle to form characters on paper (or any other recording medium). Ink-jet printing is widely used because of its low noise level, high speed, and capability of multi-color printing.
Ink-jet recording usually employs water-based ink for safety and recording characteristics. This ink contains a polyhydric alcohol to prevent nozzle clogging and ensure stable ink delivery.
The recent ink-jet printer with improved performance needs highly sophisticated ink. The most important among characteristic properties required is water resistance because printed matter has a frequent chance of it being wetted by water and beverage in offices and homes where the ink-jet printer is used.
Most of commercial aqueous dye-based ink with good color development lack water resistance almost completely and hence runs when exposed to water. This drawback is due to the fact that the dye itself is soluble in water and the polyhydric alcohol (added to ensure stable ink delivery) is also soluble in water. It is possible but impractical to omit the polyhydric alcohol in order to cope with this situation.
An attempt has been reported to make the dye itself reactive by introduction of alkoxysilyl groups so that it combines with a substrate ("Shikizai" 67 6! 356-361 (1994)). Unfortunately, this attempt is not so successful because water-soluble dyes have polar groups such as --SO.sub.3 Na, --NH.sub.2, and --CN, which react with alkoxysilyl groups introduced. Resulting products, even if obtained anyhow, are unstable in aqueous solutions and economically unfavorable.
There has been proposed a process for immobilizing a dye on a glass substrate by adding a dye to tetraethoxysilane or methyltriethoxysilane and applying the resulting sol to a glass substrate ("Shikizai" 66 9! (517-522 (1993)). This idea, however, cannot be used for aqueous ink because the sol immediately becomes gel upon addition to aqueous ink which is usually alkaline.